EmissionOn: When set to 0 it will only compute the InColour for Camera rays. This prevents it from bleeding light onto surrounding surfaces. When set to 1 it will behave as OSL intended it as act as an emissive light, which bleeds light onto surrounding surfaces.

InColour: The input colour. Can be constant or texture.

EmissionOn 0

EmissionOn 1

Note that when EmissionOn is set to 1 it will take a lot longer to compute than when EmissionOn is set to 0.